Graft copolymers based on methyl methacrylate polymers

ABSTRACT

This invention relates to a process for the manufacture of graft copolymers based on methyl methacrylate polymers. The methyl methacrylate polymer is reacted with a high molecular weight organic compound containing at least one alkylene oxide unit having an alkali metal alcoholate end group, during which reaction transesterification occurs to eliminate alkali metal methoxide and cause chain joining. The graft copolymers may be used for the production of shaped articles or coating compositions and as additives, antistatic agents or adhesion promoters for plastics materials.

This is a division of application Ser. No. 409,257 filed Oct. 24, 1973.

Polymers containing methyl methacrylate units and polyalkylene oxidegroups are transparent, tough plastics materials. They may be prepared,for example, by the method described in German Published Application No.2,023,303 involving copolymerization of methyl methacrylate withunsaturated esters of polyalkylene oxides. This method, however,requires prior manufacture and purification of the esters, this being avery complicated operation.

It is an object of this invention to provide a simple process for themanufacture of said polymers.

It is further object of the invention to manufacture polymers in whichdiolefin polymers are combined with methyl methacrylate polymers inorder to combine the high elasticity of the polydiolefin with the goodoptical properties of polymethyl methacrylate. Such polymers can bemanufactured only at very poor yields if attempts are made to effectdirect grafting of anionically polymerized polybutadiene to the livingchain end of the polymethyl methacrylate. Free-radical grafting ofmethyl methacrylate onto polybutadiene gives non-transparent products.

We have now found that graft copolymers based on methyl methacrylatepolymers may be manufactured in a simple manner by reacting a polymer ofmethyl methacrylate (A) containing at least 10% by weight of polymerizedunits of methyl methacrylate with a high molecular weight organiccompound (B) containing at least one alkylene oxide unit having analkali metal alcoholate end group and also optionally containing polymerblocks of diolefin and/or vinyl aromatic units, said reaction beingcarried out in an anhydrous inert organic solvent.

The component A is a methyl methacrylate polymer containing at least 10%and preferably more than 25% by weight of polymerized units of methylmethacrylate. It may contain usual comonomers such as acrylates ormethacrylates of alcohols having from 1 to 8 carbon atoms in the alkylradical, acrylonitrile and methacrylonitrile, and also styrene or dienemonomers. In the reaction of the invention, the methyl methacrylatepolymer is used as a solution in an anhydrous inert solvent. Thissolution may be prepared, for example, by dissolving polymethylmethacrylate in toluene or tetrahydrofuran. The traces of watercontained in the polymethyl methacrylate must be removed thoroughly.Alternatively, the methyl methacrylate polymer may be manufactured bysolution polymerization, in which case the resulting solution will beused directly. This polymerization may be initiated ionically or withfree radicals. In the case of ionic polymerization, initiators which aresuitable include lithium alkyl such butyl lithium or addition complexesof alkali metals such as cumyl potassium, naphthalene potassium andtetrameric α-methylstyrene dipotassium. Suitable free-radical initiatorsare the usual peroxides and azo compounds. Suitable solvents in bothcases are hydrocarbons such as benzene, toluene and ethyl benzene,whilst in the case of ionic polymerizations use may also be made ofethers such as diethyl ether, dioxane and tetrahydrofuran.

Component B is a high molecular weight organic compound containing atleast one alkylene oxide unit having an alkali metal alcoholate endgroup and also optionally containing polymer blocks of diolefin and/orvinyl aromatic units.

In the simplest case it is a polyalkylene oxide, preferably polyethyleneoxide or polypropylene. In this case it should have a degree ofpolymerization of at least approximately 100. It is prepared in knownmanner by polymerization of ethylene oxide or propylene oxide in,preferably, polar solvents with addition complexes of sodium orpotassium with for example α-methylstyrene, naphthalene, diphenyl,stilbene or other fused ring systems. These initiators have adifunctional action and give polymers having two living chain ends.Initiators having a monofunctional action are for example cumylpotassium, phenyl potassium and benzyl potassium. The initiators aregenerally used in amounts of from 0.001 to 0.5% by weight. Particularlysuitable solvents are ethers. Alternatively, polymerization may -particularly where initiators having a monofunctional action are used -be carried out in non-polar hydrocarbon solvents. The polymerizationtemperature is generally between -120° and +70° C.

Component B may also consist of block copolymers of polyalkylene oxideas one block with diolefin and/or vinyl aromatic polymers as the otherblock. Such block copolymers may be prepared for example as described inU.S. Pat. No. 3,050,511 or German Patent Application No. P 2,230,277 bypolymerizing butadiene or styrene either alone or together, or one afterthe other, using the said alkali metal addition complexes as initiators,whereupon ethylene oxide is added to the living chain end. Wherebutadiene is used, there are produced polybutadiene blocks having arelatively high content of 1,2-vinyl double bonds.

If organolithium compounds are used in the manufacture of the diolefinpolymers or vinyl aromatic polymers, no block copolymers are formed inthe reaction with ethylene oxide, as the growth rate of the ethyleneoxide polymerization is very low in this case. Thus an alcoholate grouptends to be formed at the chain end due to addition of one ethyleneoxide unit. Consequently, this method also falls within the scope of thepresent invention. Where butadiene is used, there are formedpolybutadiene blocks having a high content of 1, 4 double bonds,provided polymerization is not carried out in polar solvents.

Depending on whether monofunctional or difunctional initiators are usedin the manufacture of such block copolymers, the resulting polymers haveone or two living chain ends. In all cases the resulting polymers arehigh molecular weight organic compounds containing one or two alkalimetal alcoholate end groups. In the reaction of component A withcomponent B according to the present invention, this end group reactswith an ester group of the methyl methacrylate polymer to causetransesterification and elimination of alkali metal methoxide and tocause joining of the two polymer chains. This transesterification is anequilibrium reaction which, due to the excess of ester groups, isshifted well over to the grafting side. This reaction is carried out bymixing the solutions containing components A and B, preferably attemperatures of between -50° and +100° C. The reaction may take place ina variety of ways:

If component B has only one alkali metal alcoholate end group, i.e. ifit was prepared using initiators having a monofunctional action, thenpure graft polymers are formed. These are brittle but, due to the freelymobile polyalkylene oxide chain, they are water-soluble.

If, however, the component B contains two alkali metal alcoholate endgroups, i.e. if it has been prepared using initiators having adifunctional action, it can react with the methyl methacrylate polymer Aat both ends, in which case complete grafting leads to crosslinking. Theresulting products are tough and water-insoluble.

The degree of grafting may be varied by allowing the attack of of theliving alkali metal alcoholate groups to take place on more than oneester group of the methyl methacrylate polymer. This will particularlybe the case when the number of chains of component B in the reaction isgreater than that of the chains of component A. In this way it ispossible to produce graft polymers in which a number of side chains Bare grafted to a main chain A.

If, as explained above, organolithium compounds are used as initiatorsin the manufacture of the butadiene polymers, graft copolymers can beprepared which consist of

A. a main chain containing more than 25% and preferably more than 50% ofpolymerized units of methyl methacrylate and

B. at least one and preferably from two to twenty side chains per mainchain, which side chains contain more than 25% and preferably more than50% of polymerized units of butadiene and having 1, 4 configuration tothe extent of from 75 to 95%.

In this case, if initiators having a difunctional action are used, thereare obtained wholly or partially crosslinked graft copolymers in whichsome of the side chains form bridging chains between two main chains.

Following the reaction of the components, it is convenient to first stopthe living chain ends by adding proton-active substances such as organicacids, e.g. acetic acid. The graft polymers may then be precipitatedfrom the solution with petroleum ether, where the alkylene oxide contentis high, or with alcohol or water, where the alkylene oxide content islow. The graft copolymer is then isolated by filtration or byevaporation of the solvent.

The polymer chain A has a hydrophobic character, whilst the polyalkyleneoxide blocks B are generally hydrophilic. By varying the ratio ofcomponents A to B, it is possible to adjust the combination ofhydrophilic and hydrophobic properties in a desired manner. This formsthe basis of a number of applications.

The graft copolymers of the invention may be used for the production ofshaped articles or coating compositions. They are usually transparentand, when initiators having a difunctional action are used, they arealso tough. They may be used as additives to a number of plasticsmaterials. When monomeric organic compounds are polymerized in thepresence of graft copolymers containing diene polymers, grafting mayoccur at the double bonds. In such polymer mixtures or graft polymers,the hydrophilic polyalkylene oxide blocks suppress staticelectrification. This fact may be utilized for example when renderingsytrene or methyl methacrylate polymers antistatic and impact-resistant,in which case the antistatic agent will be present as an integral partof the polymer and thus will not be able to migrate or be dissolved out.Particularly advantageous is the low glass temperature of the dienepolymer blocks having a high 1, 4 content, since such block copolymersare rubber-elastic over a wide temperature range.

Due to their high water absorption properties, the graft copolymers aresuitable for use as coating materials on non-woven webs of fibrousmaterial. They may also be used as adhesion promoters for bondingpurposes, for example for bonding polystyrene and polymethylmethacrylate, or as compatibility promoters when blending variousplastics. The combination of hydrophilic and hydrophobic propertiesmakes the water-soluble graft copolymers suitable for use assurfactants, protective colloids or emulsifiers.

EXAMPLE 1

Grafting of monofunctionally living polyethylene oxide on polymethylmethacrylate

A. 1,500 ml of tetrahydrofuran (distilled over α-methylstyrenedipotassium) and 226 g of methyl methacrylate (distilled over calciumhydride) are placed in a vessel. This solution is mixed, at -40° C, with40 ml of a 0.5M solution of α-methylstyrene dipotassium intetrahydrofuran. Polymerization is complete after 1 hour and themolecular weight of the product is found to be 37,000 by measuring theviscosity.

B. 1,500 ml of tetrahydrofuran and 25 ml of cumyl potassium (as 0.5Msuspension in cumene) are placed in a separate reaction vessel. 220 g ofethylene oxide (distilled over lithium butyl) are added at 0° C andpolymerization is then effected for 4 hours at 40° C. There is obtained100% conversion and the molecular weight of the product is 9,500.

C. The contents of the two vessels are mixed together. It is found thatthe molecular weight of the polymethyl methacrylate is approximatelydoubled after only 25 minutes at room temperature. The addition ofmethanol causes no further precipitation of polymethyl methacrylate. Thegrafting yield is substantially 100%. The graft copolymer is soluble inmethanol and water and panels compression molded therefrom aretransparent and brittle.

EXAMPLE 2

Grafting of difunctionally living polyethylene oxide on anionicallypolymerized polymethyl methacrylate

A. 1,500 ml of tetrahydrofuran and 320 g of methyl methacrylate areplaced in a vessel and polymerization is effected at -40° C by theaddition of 30 ml of a 0.5M solution of tetrameric α-methylstyrenedipotassium, a conversion of 100% being obtained after 1 hour. Themolecular weight is 37,000.

B. 1,500 ml of tetrahydrofuran and 60 ml of a 0.5M solution oftetrameric α-methylstyrene dipotassium in tetrahydrofuran are placed ina vessel and 310 g of ethylene oxide (distilled over butyl lithium) areadded at 10° C, whereupon polymerization is effected for 4 hours byheating at 40° C. The molecular weight is 9,500.

C. The two solutions are mixed together at room temperature and stirred.After 1 hour there is obtained a soluble graft copolymer having amolecular weight of 150,000. This product crosslinks over the next fewhours but is still readily meltable. Compression-molded panels madetherefrom are transparent and very tough.

EXAMPLE 3

Grafting of difunctionally living polyethylene oxide on polymethylmethacrylate obtained by free-radical polymerization

A. 500 ml of toluene and 320 g of methyl methacrylate are polymerized atfrom 80° to 90° C for 6 hours by the addition of 1.0 g ofazodiisobutyronitrile. The molecular weight is 42,000.

B. 200 ml of tetrahydrofuran and 60 ml of a 0.5M solution ofα-methylstyrene dipotassium in tetrahydrofuran are placed in a vessel.310 g of ethylene oxide are added at 10° C and polymerization iseffected for 4 hours at 40° C. The molecular weight is 9,500.

C. These two solutions are mixed together at room temperature andstirred for 2 hours. The properties of the graft copolymer are the sameas given in Example 2.

EXAMPLE 4

Grafting of living styrene/ethylene oxide 3-block copolymer onpolymethyl methacrylate

A. 1,500 of tetrahydrofuran and 376 g of methyl methacrylate are placedin a vessel and polymerization is initiated at -40° C by the addition of6 ml of a 15% butyl lithium solution in hexane. Polymerization iscomplete after 1 hour and the molecular weight of the product is 80,000.

B. 1,500 ml of tetrahydrofuran and 80 ml of a 0.5M α-methylstyrenedipotassium solution in tetrahydrofuran are placed together in a vessel.136.5 g of styrene are then added at from 0° to 30° C, polymerizationbeing complete after a few minutes. The molecular weight of the productis 8,500.

To this solution there are added 356 g of ethylene oxide and aconversion of 100% is achieved after from 4 to 5 hours at 40° C. Theresulting 3-block copolymer has an ethylene oxide content of 73.5% byweight and a molecular weight of 29,000.

C. The two solutions are mixed together and stirred for several hours atroom temperature. The molecular weight of the graft copolymer is foundto be 210,000, the grafting yield being 100%. The degree of grafting isapproximately 4, the theoretical maximum being 6. The degree of graftingcan be forced to approximately 6 by heating the reaction, but thiscauses marked crosslinking and the products become insoluble. By degreeof grafting we mean the ratio of the number of active ends of the livingethylene oxide polymer to the number of polymethyl methacrylate chains.For example, when 3 chains of ethylene oxide polymer having 2 activeends per methyl methacrylate chain are used, the maximum degree ofgrafting is 6. The composition of the graft copolymer is as follows: 17%w/w of styrene, 40% w/w of ethylene oxide, 43% w/w of methylmethacrylate. Compression-molded panels made from this copolymer aretransparent and very tough.

EXAMPLE 5

Grafting of living butadiene/ethylene oxide 3-block copolymer onpolymethyl methacrylate

A. 1,500 ml of tetrahydrofuran and 163 g of methyl methacrylate areplaced in a vessel. Polymerization is initiated at -40° C by theaddition of 30 ml of a 0.5M solution of α-methylstyrene dipotassium intetrahydrofuran. The reaction is complete after 1 hour. The molecularweight is 42,000.

B. 1,500 ml of tetrahydrofuran and 163 g of butadiene are placed in avessel. Polymerization is initiated at -20° C by the addition of 30 mlof a 0.5M solution of tetrameric α-methylstyrene dipotassium. Thetemperature rises to 40° C and a conversion of 100% is achieved after afew minutes. The molecular weight is 35,000 and the content of 1,2-vinyldouble bonds is 65%.

To this solution there are added 17.5 g of dry ethylene oxide and themixture is stirred for 1 hour at 50° C. The molecular weight shows nosubstantial increase.

C. The two solutions are mixed together and stirred at room temperaturefor a few hours. Extraction tests carried out on the final product withhexane show that only 4% of ungrafted polybutadiene is present. Panelcompression molded from this graft copolymer are transparent andflexible.

EXAMPLE 6

Grafting of anionically polymerized polybutadiene (of high 1,4 content)on polymethyl methacrylate obtained by free-radical polymerization

A. 500 ml of toluene and 320 g of methyl methacrylate are caused toreact at from 80° to 90° C by the addition of 0.5 g ofazodiisobutyronitrile, the reaction being carried out for 6 hours. Themolecular weight is 81,000.

B. 2,000 ml of toluene and 335 g of butadiene (distilled over butyllithium) are placed in a vessel. Polymerization is initiated at 60° C bythe addition of 45 ml of a 1.5% lithium butyl solution in hexane.Polymerization is complete after 2 hours and the molecular weight is46,000, the 1,4 content being 90%.

To this living polymer solution there are added 4.5 g of dry ethyleneoxide and stirring is continued for 2 hours at 50° C. The molecularweight shows no change. Only one ethylene oxide unit reacts per livingpolybutadiene chain to give a lithium ethoxide end group. Nopolymerization of the ethylene oxide takes place under these conditions.

C. The two solutions are mixed together and stirred for 2 hours at 40°C. 45% of the ethoxylated polybutadiene reacts with methyl methacrylateunits. The average degree of grafting is 2. The molecular weight isfound to be about 140,000. The graft copolymer of butadiene and methylmethacrylate contains, on average, 2 polybutadiene chains and 1polymethyl methacrylate chain per molecule.

We claim:
 1. A process for the manufacture of grafting livingpolyalkylene oxide on methyl methacrylate polymers, wherein a polymer ofmethyl methacrylate (A) containing at least 10% by weight of polymerizedunits of methyl methacrylate is reacted with a high molecular weightorganic compound (B) consisting essentially of polyalkylene oxide withat least one alkylene oxide unit having an alkali metal alcoholate endgroup, said reaction being carried out in an anhydrous inert organicsolvent by mixing solutions of components A and B at from -50° to +100°C.